The present invention is related to novel bulk polymerized polymers which are water insoluble but are water swellable to form hydrogels which are transparent and which have excellent physical and optical properties. More particularly, this invention is related to shape retaining transparent articles obtained from such hydrogels and more particularly ophthalmic lenses such as contact lenses.
Recently considerable attention has been directed to obtaining polymers which are suitable for preparing hydrogels and particularly those hydrogels useful as contact lenses.
For example, various sparingly cross-linked polymeric hydrogels have been previously disclosed. For instance, such sparingly cross-linked hydrogels can be obtained by simultaneously polymerizing a water soluble monoester of acrylic or methacrylic acid in which the ester moiety contains at least one hydrophilic group such as a hydroxy group, and cross-linking the monomer as it polymerizes with a polyunsaturated cross-linking agent such as an ethylene glycol dimethacrylate. The amount of such cross-linking agent is usually less than about 1 mole % of the monoester. Such polymerizations are generally conducted in the presence of a redox initiator. For a more detailed discussion of these sparingly cross-linked polymeric hydrogels see U.S. Pat. No. 2,976,576 and 3,220,960 to Otto Wichterle et al, and see M. F. Refojo et al, "Journal of Applied Polymer Science", Volume 9, pages 2425-2435 (1965). It seems evident from these disclosures that it is necessary to have sparingly cross-linked polymeric hydrogels so as to obtain materials which have properties suitable for use as contact lenses. In order to obtain a sparingly cross-linked material it is usually necessary to employ extremely small quantities of the polyunsaturated cross-linking agent. Such amounts usually are less than 1 mole % of the monoester in the polymer.
One of the factors which determines the final properties of a hydrogel is the quantity of cross-linking agent employed. Since the quantity of cross-linking agent which is to be employed in these prior art materials is very small, the amount can only be varied slightly. Accordingly, it is not possible to effectively tailor the final properties of the hydrogel by varying the amount of cross-linking agent. Therefore although such polymers may have properties which provide suitable contact lenses, it is not possible to manipulate those properties to any appreciable extent by varying the quantity of cross-linking agent so as to tailor-make contact lenses possessing combinations of properties which may be desired for a particular application.
The above problem of restricting the quantity of cross-linking agent has been dealt with somewhat by Seiderman in U.S. Pat. No. 3,503,942. Seiderman discloses that amounts of cross-linking agent in excess of the sparing amount described in U.S. Pat. No. 2,976,576 can be employed in preparing hydrophilic polymers if certain quantities of certain alkyl acrylates or alkyl methacrylates are also incorporated in the polymer. Accordingly, Seiderman indicates that it is necessary to employ at least three different types of polymerizable monomers to form hydrogels suitable for contact lenses when quantities of cross-linking agents greater than the "sparing" amount disclosed in the above-mentioned Wichterle patents are used.
However, contrary to this it has been found that according to the present invention increased amounts of cross-linking agents can be present without the need of incorporating a third type of monomer as long as the materials, amounts, and type of polymerization disclosed in the present invention are observed. It has also been found, according to the present invention, that it is not necessary to employ ethylene glycol dimethacrylate or similar type of polyunsaturated material as the cross-linking agent in order to obtain hydrogels possessing those properties necessary to prepare acceptable ophthalmic lenses. Accordingly, it is possible according to the present invention to obtain hydrogels which can possess a whole range of suitable properties by correspondingly varying the relative amounts of the monomers used in the present invention. The present invention provides novel bulk polymerized copolymers obtained from water soluble monoesters of acrylic or methacrylic acid and a polyhydric alcohol, and certain hydrogel producing comonomers.